1. Field of the Invention
This invention relates generally to the field of oil well production equipment, and more particularly to a wire wrapped screen providing selective isolation for filtering sand at subterranean locations.
2. Brief Description of the Prior Art
Numerous screen designs are known in the art for excluding sand and detrita from a wellbore as water, oil, or gas is being produced from, or injected into, a subterranean location. Screens are sometimes used as the sole means of filtering sand from the fluids wherein the screen openings are sized to stop the movement of the formation sand and allow continual flow of fluid into the wellbore. Gravel or other particulate matter is also used in conjunction with screens such as "gravel pack" screens wherein the gravel or particulates are sized to restrain movement of the formation sand and the screen openings are designed to restrain the movement of the gravel or particulates to allow continual flow of fluids therethrough.
Common well liner screens employ a base pipe having a plurality of openings through the sidewall which is surrounded by longitudinally extending spacer bars, rods, or rings and over which a continuous wire is wrapped in a carefully spaced spiral configuration to provide a predetermined constant gap between the wire wrapping. The gap allows fluids to flow through the screen and retains the movement of particulate materials such as sand or gravel. Such a screen is commonly referred to as a "wire wrapped" screen.
Most "wire wrapped" screens have the common feature of an "inner annulus" or space between the base pipe and the wire wrapping. The "inner annulus" is desirable for the purpose of allowing free transmission of fluids that flow through the wire wrapping gaps into the openings of the base pipe. The absence of the inner annulus would significantly restrict fluid flow rates.
The inner annulus in a wire wrapped screen causes several serious problems during installation of a gravel pack and the stimulation of a screen or gravel pack. Problems are associated with bypassing of fluid through the inner annulus as the gravel is being transported down the outer annulus or space between the wire wrapping and wellbore and bypassing of fluid being injected, or produced into or from a screen or gravel pack.
Bridges of gravel are in part created, or enhanced, as fluid that carries the gravel flows into the inner annulus causing an effective increase of gravel concentration in the fluid that remains in the outer annulus because the gravel cannot enter through the wire wrapping but fluid can do so freely. If the gravel concentration increases beyond a critical magnitude it will form a bridge which wedges in the outer annulus and halts further movement of gravel before the outer annulus is fully packed with gravel.
Another problem caused by fluid freely entering the inner annulus from the outer annulus during gravel packing highly deviated wellbores is commonly referred to as "duning". In wellbores having angles of from 45.degree. to 90.degree. from vertical, especially those requiring the gravel to be packed along intervals ranging from several feet to more than a thousand feet, the gravel tends to fall to the low side of the wellbore by gravity as the gravel is transported by a carrying fluid. As gravel accumulates, the fluid is diverted to the high side of the wellbore and into the inside of the screen, thereby reducing the velocity of carrier fluid flow in the outer annulus and the capability of the fluid to force gravel toward the bottom, or end, of the outer annulus.
Gruesbeck et al, U.S. Pat. No. 4,046,198, attempts to solve this problem by providing a means of inserting a wash pipe in the base pipe to reduce the ease with which fluid can flow into the pipe. Maly et al, U.S. Pat. No. 3,637,010 teaches the use of flexible baffles to restrict the flow of fluids into the base pipe. However, both patents ignore the effect of the inner annulus between the base pipe and screen. Other relevant patents are: Jones, U.S. Pat. No. 3,428,128; Van Westrum, U.S. Pat. No. 2,155,744; Gurley et al, U.S. Pat. No. 3,789,927; Burns, U.S. Pat. No. 2,942,664; and Chancellor et al, U.S. Pat. No. 3,153,451.
Another problem occurs in wells where voids, or areas where no gravel was packed in the outer annulus, are found in the gravel pack. Such voids will allow sand to be produced freely to the screen, in some cases through the screen, and in other cases causing severe erosion of the screen and ultimate failure of the screen to stop sand incursion. One method to solve the problem of voids without having to remove the screen from the wellbore is to wash the gravel pack with a downhole washing tool. This will sometimes disturb the gravel arrangement sufficiently to settle the pack and fill the void spaces. However, the effect of the washing operations is reduced by free flow of fluids vertically up or down the inner annulus instead of radially out into the gravel. Thus some, or most, of the washing fluids will bypass the gravel and restrict the turbulent action necessary to rearrange the gravel located in the outer annulus.
Other problems are caused during attempts to stimulate a well that has a screen or a screen with a gravel pack, as the stimulation fluids are difficult to inject into the portion of the screen or gravel pack where the stimulation fluid is needed. Common stimulation fluids are hydrochloric acid, a combination of hydrochloric acid and hydrofluoric acid, diesel oil, kerosene, aqueous solutions, dispersions or emulsions of surfactants, and the like, which are injected into the screen or gravel packed wellbore via the inner pipe.
The stimulation fluids are forced out through the screen, gravel, and into the subterranean formation to dissolve, or remove, materials that are plugging or restricting the flow of fluids. Such plugging materials are commonly carbonaceous, siliceous, or organic solids and the like, which will be attacked or dispersed physically or chemically, and pushed away from the wellbore and subsequently flowed out of the wellbore with the produced fluids. Since the stimulation fluids must adequately contact the plugging materials to effectively remove them, it is imperative that the stimulation fluids be injected into the areas where the plugging materials have mostly accumulated. However, the stimulation fluids follow the path of least resistance into the portions of the gravel packed well bypassing the portions of the formation or gravel pack which are most severely plugged.
A common method of combating this problem is to use a special injection tool arrangement that mechanically isolates short sections of the screen or gravel pack inside the base pipe of the screen. This helps force some of the stimulation fluids into directed portions of the screen or gravel pack, but the inner annulus cannot be isolated by such a tool, and allows much of the injected stimulation fluids to bypass away from the intended zone.
Woodruff, U.S. Pat. No. 2,796,939 discloses a well liner having compartments prepacked with gravel which is lowered into the wellbore as an integral unit and in which fluid backwashed downwardly therethrough is directed outwardly through the gravel.
Even when injection isolation tools are employed, injection fluids such as steam, water, and the like, are transferred to zones where these fluids are not needed. When unwanted water or gas is being produced from a part of the well within the gravel packed interval and a sealing chemical or material is injected into the gravel pack to seal the gravel and stop the production of the unwanted fluid, the sealing chemical may bypass the desired zone via the inner annulus.
The present invention is distinguished over the prior art in general, and these patents in particular by a well liner having a selective isolation screen including an elongate base pipe having openings therethrough which is surrounded by circumferentially spaced, longitudinally extending spacer bars, one or more annular cylindrical sleeves secured to the exterior of the base pipe and a continuous wire wrapped therearound to form an inner annulus isolated at predetermined vertical locations corresponding to specific well requirements of varying lengths and production formations within the wellbore. The continuous wire is wrapped over the spacer bars and sleeves in a spaced spiral configuration to provide a predetermined constant gap between the wire wrapping. The gaps in the wire wrapping are sized to stop the movement of the formation sand and allow continual flow of fluid into the wellbore. Gravel or other particulate matter may be pumped down the borehole in the outer annulus between the interior of the borehole or an outer casing and the outer surface of the wire wrapping. The gravel and other particulates are sized to restrain movement of the formation sand and the screen openings or gaps are designed to restrain the movement of the gravel or particulates to allow continual flow of fluids therethrough.